As a horizontal radiation antenna of the related art, in W. R. Deal, N. Kaneda, J. Sor, Y. Qian, and T. Itoh, “A New Quasi-Yagi Antenna for Planar Active Antenna Arrays”, IEEE Trans. Microwave Theory Tech., June 2000, Vol. 48, No. 6, pp. 910-918 (hereafter, “Doc 1”), a configuration is described in which, while a feeding line, an unbalance-balance converter electrode (hereinafter, referred to as a balun electrode), a radiation element, a passive element, and the like are formed on the front surface of a dielectric substrate, a grounded conductor plate is formed on the back surface of the dielectric substrate.
In addition, in Japanese Unexamined Patent Application Publication No. 6-204734 (hereafter, “Doc 2”), a configuration is described in which, while a microstrip line used for power feeding and a conductive body cover are provided on the front surface of a dielectric substrate, a grounded conductor plate is provided on the back surface of the dielectric substrate. In this case, the leading end portion of the microstrip line is located on the end portion side of the dielectric substrate and electrically connected to the grounded conductor plate. In addition, while the conductive body cover is formed in a substantially box shape, one end side of which is open, and surrounds the leading end portion of the microstrip line, the peripheral portion thereof is electrically connected to the grounded conductor using a plurality of conductor pins. In addition, in cooperation with the end edge of the grounded conductor plate, the conductive body cover configures a slot whose length is about a half wavelength in a direction parallel to the dielectric substrate.
Furthermore, in Japanese Unexamined Patent Application Publication No. 2007-311944 (hereafter, “Doc 3”), a configuration is described in which, while, on the front surface of a dielectric substrate, a ground electrode is provided that has a notch portion whose end portion side is open, a feeding electrode is provided within the notch portion of the ground electrode. In this case, a slot line is formed owing to the outer peripheral edge of the feeding electrode and the inner peripheral edge of the ground electrode.